almost everything works fine, with one minor exception: In the xml file with the virtual machine description for libvirt (/etc/libvirt/qemu/centos62-xxx.xml), the disk image format is raw instead of qcow2:

<domain type='qemu'>

...

<devices>

<emulator>/usr/libexec/qemu-kvm</emulator>

<disk type='file' device='disk'>

<driver name='qemu' type='raw'/>

I have to manually correct this before I can start the virtual machine.